Life Sciences: Year In Review 1993

ZOOLOGY

Cannibalistic salamanders, social structures of frogs and pilot whales, and warm-blooded fish were all involved in zoological advances in 1993. In addition, studies of the fossil record challenged traditional theories regarding the origin of avian flight and the ancestry of humans.

Studies on cannibalism in salamanders and on the social structure of whales provided support for theories of kin selection, the tendency to favour genetic relatives over unrelated individuals, by revealing situations in which animals modify their behaviour when they belong to a genetically related family unit. The larval young in some populations of tiger salamanders (Ambystoma tigrinum) are known to become cannibalistic, feeding on other tiger salamanders. The cannibals grow larger than noncannibalistic larvae and develop specialized structures in the mouth that aid in eating other salamanders. Cannibalism occurs most frequently when larvae develop under crowded conditions. David W. Pfennig of Cornell University, Ithaca, N.Y., and James P. Collins of Arizona State University discovered that tiger salamanders reared in genetically unrelated groups are more likely to develop into cannibals than are salamanders raised in groups of siblings. They conducted experiments in which similar-sized larvae were placed in various groups, some being all siblings and some being unrelated. All larvae were of similar size so that variation in body size could not be used by the larvae as a cue to whether individuals were related. The investigators hypothesized that larval salamanders release chemical cues that can be used to distinguish close kin, which have a similar "smell," from unrelated larvae.

Kin-selection theory was also supported by evidence that individual organisms can increase their own genetic success by curtailing breeding and possibly helping their relatives. In a study of the biology of long-finned pilot whales in the Faeroe Islands, southeast of Iceland, Bill Amos of the University of Cambridge and colleagues Christian Schlötterer and Diethard Tautz of the University of Munich, Germany, found that males exhibit atypical behaviour. Pilot whales form large social groups called pods. The investigators used molecular techniques to establish that pod members were closely related, forming an extended family. A pod normally has more adult females than males and may number more than 100 individuals. In most mammalian species in which females live in groups, genetic inbreeding is avoided when male offspring disperse from their homesite before they become breeding adults. Pilot whale males remain with their family pod, yet genetic studies revealed that males in a pod rarely or never breed with the females, who might be their mothers or sisters. Mating is presumably carried out when different pods encounter each other in the ocean. Whether or how the nonbreeding males contribute to the welfare of their relatives in a pod remained to be learned, but defense from marine predators or assistance in a communal feeding effort was suggested. Ironically, the cohesive family structure of long-finned pilot whales makes them prey to human whale hunters. In the early 1990s about 1,700 of the whales were killed each year because pods could easily be herded into coastal areas.

Evidence of the way in which mating systems can develop in the best interest of an individual but not necessarily of the species was presented by Godfrey R. Bourne of Florida Atlantic University in studies of the mating system of a tropical frog, Sinax rubra, in Guyana. If given a choice, females, regardless of body length, select smaller male mating partners, usually about 80% of their own size. Experiments revealed that this size ratio of female to male produced the highest rate of fertilization of a female’s eggs. During mating, a male frog clasps a female and releases sperm while she deposits eggs in the water. A male frog that is larger than the female is not in the proper position for the sperm to reach all of the eggs; thus, he fertilizes significantly fewer. Males smaller than the optimal size do not have enough sperm to fertilize all the eggs. Therefore, to maximize egg fertilization and have the highest reproductive success, a female needs a mate of the proper size and so chooses one accordingly.

A larger male frog, however, often displaces the smaller one that is chosen by the female and ends up mating with her instead. Competition between a small male that is preferred by the female and a large male intruder can reduce female reproductive success because fewer eggs are fertilized. On the other hand, the breeding success of the larger male is enhanced. Smaller males sometimes successfully mate by remaining quiet but alert for approaching females. When a female passes by on her way to check out a calling male, a small, silent "satellite" male may intercept her and mate. This competition among males can reduce the reproductive success of a particular female but ensure the propagation of a particular male’s genes.

Most of the world’s fish species, along with reptiles and amphibians, are ectotherms, or cold-blooded animals, having body temperatures corresponding to that of their surroundings. Endothermic, or warm-blooded, animals have the ability to elevate body temperature internally. The trait is characteristic of mammals and birds as well as some sharks and certain marine fish, including mackerels, tunas, and billfishes (e.g., marlins and swordfish). Two contrasting theories exist to explain what selection pressures were influential in the evolution from ectothermy to endothermy. One theory proposes that endothermy arose following selection for a capability to maintain stable body temperatures across a broad range of environmental temperatures, permitting exploitation of varying thermal conditions. The other proposes that endothermy evolved in response to selection for an increase in aerobic capacity (ability to use oxygen) associated with higher metabolism and a more active lifestyle.

Barbara A. Block and colleagues of the University of Chicago used techniques of molecular genetics to establish the phylogenetic relationships among ectothermic fish species and the three groups of endothermic fishes. They found mackerels, tunas, and billfishes each to be more closely related to ectothermic species than to each other, documenting that endothermy evolved independently in the three different groups. In some species, such as the butterfly mackerel and swordfish, warming is restricted to the central nervous system and retina. The phylogenetic distribution and variable expression of endothermy among the fish groups led the researchers to conclude that endothermy in fishes evolved in response to the advantages of expanding into habitats of varied temperatures, not to a requirement of increased aerobic capacity.

A new living species of large mammal, the first such in more then 50 years, was identified from a physical examination and molecular analysis of skulls, teeth, and skins collected from a largely unexplored rain forest in Vietnam’s mountainous central neck. The animal itself, however, had yet to be seen alive by the scientists involved at the time their findings were published. John MacKinnon of the Asian Bureau for Conservation, Hong Kong, and colleagues of the Vietnamese Ministry of Forestry placed the animal in the Bovidae family, which includes cattle, goats, sheep, and antelopes, and described it as weighing about 100 kg (220 lb) as an adult and having a rich brown coat with white and black markings and sharp, straight horns up to 52 cm (20 in) in length. The new bovid, called the "forest goat" or "spindle horn" by local Vietnamese hunters, was given the name Pseudoryx nghetinhensis.

Two major theories have been proposed for the evolution of flight in birds. One is that flight evolved in ground-dwelling animals that were preadapted for flight. The other is that flight originated in tree-dwelling species. On the basis of fossils from the Late Jurassic (150 million years ago), Archaeopteryx is the generally accepted predecessor of flying birds and the focus of most theoretical discussions on the origin of avian flight. During the year Alan Feduccia of the University of North Carolina lent credence to the origin of flight from tree-dwelling forms by means of a study that compared the claw geometry of Archaeopteryx with that of modern birds. The claw curvature of the ancient bird was shown to be similar to that of modern birds that perch in trees or climb tree trunks rather than to that of ground-dwelling birds, suggesting that this earliest known feathered ancestor of birds was arboreal. (See Ornithology, below.)

Among the most controversial evolutionary interpretations from the fossil record are those surrounding the relationships between humans and other primates. David R. Begun of the University of Toronto examined fossil hominids from Hungary estimated to be 10 million years old. He concluded that they may be the closest known relatives of chimpanzees, gorillas, and humans. In addition, his findings supported the view that humans are more closely related evolutionarily to chimpanzees than either are to gorillas, a position held by many molecular biologists.

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